There have been many remarks made about how independent the two engine control systems are:
- Separate power supplies
- Separate computers
- Separate almost everything
However, I believe it is highly likely that both computers were running the same software. If the software were fed the same inputs, and there was an error in the software, they could both output the same erroneous controls to the engines. I'm not saying that this is what caused the crash. What I'm saying is that just because everything appears to be physically separate does not mean that they could not fail simultaneously from the same fault, if that fault is a software fault.
I have worked professionally in the software development business for thirty-eight years, and I have personally seen this very type of software fault cause the failure of many online banking systems. It is very difficult and expensive, but worthwhile, to avoid. Many persons do not realize how dependent their very lives have become on computer software and silicon today.
On the space shuttle, there are three main computers which handle operations and decision-making processes. NASA had them independently programmed in three different languages by three independent teams of software developers, but to the same specifications in terms on inputs and outputs to avoid this very issue. In case of disagreement, two of the three computers could vote to over-rule the third computer output.